Conventionally, a technique of impregnating pores of a porous material with an impregnating liquid, such as a liquid or a liquid (suspension) in which solid particles are suspended, has been widely industrialized. For example, prepreg, which is used for a printed-wiring board and the like, is produced in such a way that (i) bubbles in a fiber substrate is removed, (ii) the fiber substrate is impregnated with liquid resin, and (iii) the fiber substrate thus impregnated is thermally dried so that the liquid resin is half-cured.
As an example of such a method, Patent Literature 1 discloses a resin impregnation method in which a fiber substrate is impregnated with liquid resin by (i) sandwiching the liquid resin between the fiber substrate and a non-permeable sheet, which is wound around a roll and which the liquid resin does not permeate, and (ii) pressuring the liquid resin from a non-permeable sheet side. However, according to the resin impregnation method, bubbles remain in prepreg because the fiber substrate is pressure-impregnated with the liquid resin. Therefore, there has been a problem that it is not possible to sufficiently impregnate (fill) a fiber substrate with a liquid or a suspension.
Therefore, it is more effective to use a vacuum impregnation method than a pressure impregnation method in order to sufficiently remove bubbles (air bubbles) in pores of a porous material and to sufficiently impregnate the pores with an impregnating liquid. Techniques using this vacuum impregnation are disclosed in, for example, Patent Literatures 2 and 3.
For example, Patent Literature 2 discloses a negative pressure acting device which carries out a negative pressure acting process with respect to individually separated objects (porous materials) in such a way that (i) an acting liquid stored in a liquid storing tank is raised into a negative pressure producing tube, (ii) a negative pressure space is produced in an upper part of the negative pressure producing tube by means of a balance between the acting liquid in the negative pressure producing tube and an atmospheric pressure on the acting liquid in the liquid storing tank, and (iii) the objects are reciprocated up and down between a lower end opening of the negative pressure producing tube and the negative pressure space. According to the technique disclosed in Patent Literature 2, a lower on-off valve which opens and closes the lower end opening is provided near the acting liquid in the liquid storing tank so as to produce the negative pressure space.
Further, Patent Literature 3 discloses a method of impregnating a film-shaped material (sheet-shaped material) with an impregnating liquid in which method (i) the film-shaped material is vacuum-deaerated in a state where the film-shaped material is hold in a vacuum tank and (ii) the film-shaped material is moved in the impregnating liquid with the use of a pair of seal rollers so as to be impregnated with the impregnating liquid.
Further, as a conventional vacuum impregnation process, there is known a batch process. According to the batch process, for example, rolls obtained by individually rolling up porous materials are put in an airproof pressure-resistant container, and then the airproof pressure-resistant container is deaerated by suctioning air in the airproof pressure-resistant container with the use of a vacuum pump or the like so that the airproof pressure-resistant container has a negative pressure in it. Thereafter, an impregnating liquid is introduced into the airproof pressure-resistant container, and then the negative pressure in the airproof pressure-resistant container is returned to an atmospheric pressure. It is thus possible to impregnate pores of each of the porous materials with the impregnating liquid with the use of a differential pressure, serving as a driving force, between an internal pressure (negative pressure) of the pores of each of the porous materials and the atmospheric pressure. Note that, in the batch process, the impregnating liquid can be put in the airproof pressure-resistant container in advance before carrying out the deaeration by suction.
Note that Patent Literature 4 discloses a deaerator which vacuum-deaerates a liquid-like body by producing a vacuum in accordance with the Torricelli vacuum experiment (principle of Torricelli's vacuum). More specifically, the liquid-like body is vacuum-deaerated by (i) closing a valve so that a depressurizing pipe, an injection pipe, and an exhaust pipe are filled with the liquid-like body and then (ii) opening the valve so that the liquid-like body flows into the vacuum produced in an upper part of the depressurizing pipe.